A woman holds her child outside a church near Sandy Hook Elementary School, where officials said 20-year-old Adam Lanza killed 26 people, including 20 children, as youngsters cowered in their classrooms and trembled helplessly at the sound of gunfire. Officials said Lanza, armed with two handguns, committed suicide. They said another person was found dead at a second scene, bringing the death toll to 28.

Students Eric Harris, 18, and Dylan Klebold, 17, opened fire at Columbine High School in Littleton, Colo., killing 12 classmates and a teacher and wounding 26 others before killing themselves in the school’s library.

Photo: Kin Man Hui

7. April 20, 1999, Columbine High School in Littleton, Colo.

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7. April 20, 1999, Columbine High School in Littleton, Colo.... Photo-3892936.53915 - SFGate

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8. April 3, 2009, Binghamton, N.Y.

Jiverly Wong, 41, killed 13 people at the American Civic Association immigration center, then himself.

Thirteen soldiers and civilians were killed and more than two dozen were wounded when a gunman walked into the Soldier Readiness Processing Center and opened fire. Army psychiatrist Maj. Nidal Hasan is charged with 13 counts of premeditated murder and 32 counts of attempted premeditated murder.

Army veteran Howard Unruh gunned down 13 people while walking around his neighborhood. He was declared a paranoid schizophrenic and sent to a psychiatric hospital without a trial. He died in 2009 at a Trenton, N.J., nursing facility.

Twelve people were killed when a gunman entered a movie theater, released a gas canister, then opened fire during opening night of the Batman movie “The Dark Knight Rises.” James Holmes, a 24-year-old former graduate student at the University of Colorado, has been charged in the deaths.